Inorganic glass exhibits high transparency, high heat-resistance, and dimensional stability, and therefore, has long been used in a wide range of industrial fields as a structure that passes visible light while dividing space, without impairing visibility. Despite these excellent characteristics, inorganic glass has three major drawbacks: a large specific gravity of 2.5 or more; low impact resistance, i.e., easy to break; and an unexpectedly high cost for lens processing. Particularly because of the recent progression of downsizing such as reductions in weight and thickness in various industrial fields, there is an ever increasing demand from users to overcome the above-mentioned drawbacks.
Transparent thermoplastics and thermosetting plastics are considered to be promising materials that can meet this industrial demand. Examples of transparent thermoplastics include PMMA (polymethyl methacrylate), PC (polycarbonates), and the like. Among these, PMMA is also referred to as an organic glass, and is attracting attention as a material that exhibits excellent transparency and overcomes two of the drawbacks of the glass. These transparent plastics, however, are significantly inferior to inorganic glass in heat resistance, and can only be used in limited applications.
Examples of transparent thermosetting plastics include epoxy resins, curable (meth)acrylate resins, silicone resins, and the like. These thermosetting plastics are generally more resistant to heat than the above-mentioned thermoplastics. Among the above, epoxy resins, in particular, have been generally used as materials or adhesives for encapsulating LEDs. However, with the recent attention being paid to white LEDs, yellowing and cracking of epoxy resins caused by heat or ultraviolet light have been posed as new problems. Moreover, molded articles of epoxy resins have the drawback of having low impact resistance and being brittle. On the other hand, curable (meth)acrylate resins have an excellent balance of heat resistance and moldability, excellent balance of physical properties of the molded article, etc.; however, they have the drawback of having high water absorption, which is accompanied by a large dimensional change.
Among thermosetting plastics, silicone resins exhibit excellent resistance to heat, weather, and water. Therefore, silicone resins solve the above-mentioned problems of various plastics, and are currently the most promising materials as an alternative to inorganic glass. For example, the use of silicone resins as mold members for LEDs (Patent Literatures 1 and 2), color filter materials (Patent Literature 3), and the like has been suggested. The use of these general silicone resins, however, has posed problems such as cracking due to thermal shock, complicated steps of processing the resins into a lens shape, and the like.